The present invention relates to the field of acoustic devices. In particular, the present invention relates to a noise suppression system for use in noisy environments.
Many environments have unwanted noise. For example, factory machinery, aircraft engines, motor vehicle traffic and the like generate noise levels that can be detrimental to hearing and interfere with voice communication.
The primary component of unwanted noise is the direct sound wave d(t) from the noise source. The secondary component of unwanted noise is the echo of the direct sound wave off a reflecting surface, such as the exterior surface of a building or an interior wall. In open environments, noise is primarily direct noise. In a typical environment, there is some reflection (echo) of the primary sound wave off buildings or walls, which adds reflected noise to the original direct noise. In confined enclosed environments the interior surfaces and surfaces of objects contained inside the enclosed environment generate multiple echoes r(t) of the same sound wave. Multiple echoes of the reflected sound wave combined with the direct sound wave d(t) is the noise s(t) captured by the microphone.
One example of an environment in which reverberation noise is considered a significant problem is mining. Mines are typically located underground in closed quarters surrounded by reflecting walls of substantially homogenous materials. Powerful mining equipment generating acoustic waves is used on a daily basis. The noisy environment makes voice communication between mine workers very difficult. In addition, the accumulation of direct acoustic waves and their reverberation from the inner surfaces of the mine tunnel and other mining equipment in the tunnels leads to a high noise level detrimental to the ear. The risk of hearing loss after long exposure to high ambient noise levels has been well documented.
Various devices have been proposed to reduce noise levels. One of the most direct means for reducing the sound intensity is to surround the source of the noise with acoustic baffles. Such baffles placed on or in front of reflecting walls and other objects, cut off the reflecting acoustic propagation path. Various absorbing materials dissipate incident sound energy by converting it to heat energy. Sound absorbers work well for the high frequency range. However, acoustic baffles are bulky and do not work well for low frequencies. In certain industrial environments such as mining, acoustic baffles are not practical.
Active noise cancellation, where the cancellation of noise is sought by emitting an artificial sound to cancel the unwanted sound, is known. An active noise cancellation system uses a microphone, an amplifier and a loud speaker, in an arrangement to cancel the sound in a particular area, typically the area in the vicinity of an operator. The microphone provides a measure of the noise in a local area relatively distant from the direct noise source. The amplifier drives the loudspeaker to produce equal amplitude and opposite phase acoustic signal to cancel out the sound in the local area. Although a significant sound reduction is experienced, it is experienced only for that particular area and no other areas where the sound may be equally objectionable. In addition, such an arrangement is prone to the production of interference patterns, which may even increase the noise intensity in other locations.
A variation of the above system includes a second microphone disposed at a noise receiving point. The output of the second microphone is a measure of the cancellation error, which is used to adjust the coefficients of an adaptive filter in a closed loop recursive system to further reduce the noise received at the second microphone.
In another type of active noise cancellation system a microphone is placed very close to the acoustic noise source, which is approximated a point source. The signal processing circuit produces a phase opposition signal, which is adjustable by adjusting the distance between the microphone and the loudspeaker. Such systems are restricted to a point source of acoustic radiation of a single frequency, and do not work well when the noise is produced by large vibrating surfaces that may be vibrating in a complex mode to produce a wide spectrum of frequencies.
Another type of active noise cancellation system uses a pair of microphones and a headset worn by the operator. A first microphone picks up a first sample of the background noise. A second microphone placed some distance away from the first microphone, picks up a second sample of the background noise. To cancel the noise, the signal from the second microphone is processed in an adaptive filter and combined in opposite phase relationship to the signal from the first microphone. The processed second signal from the second microphone tends to cancel the noise signal arriving at the first microphone. The headset actively reduces the level of noise reaching the ears, thereby providing ear protection for workers when worn in high noise areas. However, such headsets prevent workers from hearing alarm signals and block speech communication between workers.
In general, prior art noise cancellation systems do not work well in relatively high background noise environments with complex reverberating structures especially in confined spaces, such as are commonly found in the mining industry.
In addition to ear protection, noise suppression systems are used in communications systems to help workers hear speech signals in noisy environments. Noise reduction communication systems distinguish the desired speech component from the background noise component of the combined signal. By canceling or reducing the background noise component, the signal-to-noise ratio is increased thereby enhancing the quality of the received speech.
One type of noise suppression system uses a pair of microphones connected to a headset worn by the operator. A first microphone (for voice) picks up a first signal containing the intended speech plus the background noise. A second microphone (for noise) placed some distance away from the first microphone, picks up a sample consisting mostly of the background noise and less of the speech signal. The signal from the second microphone (background noise) is processed in an adaptive filter and subtracted from the signal from the first microphone (speech plus the background noise) to reduce or cancel the background noise component of the first signal.
Since the second microphone is placed some distance away from the first microphone, the background noise sample (at the second microphone) is not exactly the same background noise signal that is arriving at the first microphone. The function of the adaptive filter is to compensate for the difference in acoustic paths of background noise arriving at the first and second microphones.
U.S. Pat. No. 5,754,665 to Hosoi shows a dual noise canceller with dual microphones and dual adaptive filters intended for use in an automobile telephone speaker system. First and second microphones are placed near the driver and passenger, respectively. When one microphone is used for conversation, the other microphone is used for collecting noise, and vice versa. A noise-reduced version of the first voice signal is obtained by using one of the adaptive filters. When the second microphone is used for conversation, the first microphone is used for collecting noise. A noise-reduced version of the second voice signal is obtained by using the second adaptive filter. The two noise reduced versions are added to form the outgoing telephone voice signal.
In order to cancel unwanted noise, it is necessary to obtain an accurate estimate of the noise to be cancelled. In an open environment, where the noise source can be approximated as a point source, microphones can be spaced far apart as necessary and each will still receive a substantially similar estimate of the background noise. However in a confined environment containing reverberation noise caused by multiple sound reflections, the sound field is very complex and each point in the environment has a very different background noise signal. The further apart the microphones are, the more dissimilar the sound field. As a result, it is difficult to obtain an accurate estimate of the noise to be cancelled in a confined environment by using widely spaced microphones.
If the two microphones are moved closer together, the second microphone should provide a better estimate of the noise to be cancelled in the first microphone. However, if the two microphones are placed very close together, each microphone will cause an additional echo to strike the other microphone. That is, the first microphone will act like a speaker (a sound source) transmitting an echo of the sound field striking the second microphone. Similarly, the second microphone will act like a speaker (a sound source) transmitting an echo of the sound field striking the first microphone. Therefore, the signal from the first microphone contains the sum of the background noise plus a reflection of the background noise, which results in a poorer estimate of the background noise to be cancelled.
The present invention is embodied in a dual microphone noise suppression system in which the echo between the two microphones is substantially canceled or suppressed. Reverberations from one microphone to the other are cancelled by the use of first and second line echo cancellers. Each line echo canceller models the delay and transmission characteristics of the acoustic path between the first and second microphones.
The present invention is further embodied in an ear set to be worn in the outer ear. The ear set is a self-contained molded unit, with integral dual microphones, battery, ear canal speaker, signal processing electronics that is convenient to wear and will not interfere with communication between workers or physical activity while working.
In a first embodiment, a noise suppression system in accordance with the present invention acts as an ear protector, canceling substantially all or most of the noise striking the dual microphones of the ear set. In a second embodiment, a noise suppression system in accordance with the present invention acts a noise suppression communication system, suppressing background noise while allowing communication signals to be heard by the wearer.